Single integrated services digital network (ISDN) lines provide an intermediate solution for users in need of digital services having a capacity greater than that of conventional telephone subscriber lines, but less than that of a T1 connection. PBX systems for managing the connection of a plurality of ISDN terminals to one or more ISDN lines are known to the art. The Bellcore National Standard (NI-2) sets out a system that supports 8 terminals, each with it own "telephone number" on a single basic rate interface (BRI). Typically, a single BRI line is used to connect a central office to a network terminator. The network terminator is then connected to each of up to eight terminals. Any of the 8 terminals can be connected to the BRI through the network terminator. The central office assigns an individual "telephone number" to each terminal. Since a BRI line has two B-channels that can be connected to different terminals, this arrangement allows any two of the eight terminals to receive or make calls via the network terminator simultaneously.
While this arrangement is well suited for an 8 terminal system on a single BRI line, difficulties arise when one wishes to connect several BRI lines to a PBX which services more than 8 terminals. Consider an arrangement in which a PBX is connected to 3 BRI lines, and the PBX connects these lines to 24 terminals. From the telephone central office's point of view, the PBX appears to be three separate network terminators, each connected to 8 terminals. Consider three network terminals that the central office views as being connected to the same 8 terminal group. If two of these terminals are using the corresponding BRI line and a call comes for the third terminal, the caller will be given a busy signal, even if the other BRI lines connected to the PBX are free.
One method for preventing this apparent busy line situation is to utilize electronic key telephone systems (EKTS) groups. All of the terminals in an EKTS group are notified of an incoming call for any one of the terminals in the group. In the normal utilization of this feature, the inbound call rings on the identified terminal; however, the call is also displayed without ringing on the other terminals in the group. This feature was originally implemented to allow for phone backup within an office work group. Each member of the work group would be placed in the same EKTS group. The terminals of all members display all of the numbers associated with the group. An inbound call for one of the members of the work group would then be displayed on the telephones of all of the members of the work group. If the intended party did not pickup the call, one of the members of the work group could answer the call from his or her phone.
In the case of the above described PBX, all 24 terminals would be defined as being part of the same EKTS group. Hence, each time a call is received for a terminal on a busy BRI line, the PBX will still receive call setups for the work group members on the unused BRI lines. The PBX can then connect the call to original terminal, if that terminal is free, by emulating a connection to one of the terminals the central office identifies with a free BRI line.
While this technique solves the problem of allowing any terminal to use any BRI line connected to the PBX, the PBX must deal with 24 call setups each time an inbound call is received. The time needed to process these call setups is significant and places undesirable constraints on the PBX controller.
Broadly, it is the object of the present invention to provide an improved ISDN PBX system.
It is a further object of the present invention to provide an ISDN PBX system that allows any terminal connected thereto to be connected to any ISDN line connected thereto without the need to receive one call setup for each terminal each time a call is received for any terminal connected to the PBX.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.